Hitherto the ring-opening polymerization of an alkylene oxide (AO) is the formation of a linear polymer by ring-opening a cyclic compound having high reactivity and shown by the following scheme:

This epoxide having an oxygen heteroatom in the ring has a high polarity and becomes a reaction-active site. Most of the reactions proceed by an ionic mechanism (ionic polymerization). The ionic polymerization can be classified on the basis of the reaction mechanism into an anionic polymerization, a cationic polymerization and a coordination polymerization.
The anionic polymerization has been historically studied for very many years and has been industrially widely used for the purpose of producing a polyalkyleneglycol derivative.
The cationic polymerization is industrially used for the purpose of obtaining a polymer having a relatively low molecular weight (at most about 5,000). It is known that the cationic polymerization has a lower conversion ratio at the polymerization and easier formation of by-products than the anionic polymerization.
The coordination polymerization is called also as coordination anionic polymerization and exhibits a behavior which is intermediate between the anionic polymerization and the cationic polymerization. The coordination polymerization has one type of coordination mechanism off the anionic and cationic polymerizations and can give a polymer having a high molecular weight (molecular weight of at least 100,000). However, the coordination polymerization has the defects that the conversion ratio at the polymerization is relatively low, that the operation of catalyst is associated with danger, and that the handling is difficult.